In general, a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move into a position where the inductance of an energized phase winding is maximized. In one type of reluctance machine the energization of the phase windings occurs at a controlled frequency. These machines are generally referred to as synchronous reluctance machines. In another type of reluctance machine, circuitry is provided for detecting the position of the movable part (generally referred to as a "rotor") and energizing the phase windings as a function of the rotor's position. These types of machines are generally known as switched reluctance machines. The present invention is applicable to both synchronous and switched reluctance machines.
The general theory of the design and operation of reluctance machines in general, and switched reluctance machines in particular, is known in the art and is discussed, for example, in Stephenson and Blake, "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives", Presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
As explained above, the basic mechanism for torque production in a traditional reluctance motor is the tendency of the rotor to move into a position to increase the inductance of the energized phase winding. In general, the magnitude of the torque produced by this mechanism corresponds to the magnitude of the current in the energized phase winding such that the motor torque is heavily dependent on the phase current waveforms. For an ideal traditional reluctance motor with no magnetic saturation, the instantaneous torque T, per phase, is: ##EQU1## Where i is the instantaneous current in the energized phase winding and dL/d.theta. is the derivative of the phase inductance L with respect to the rotor position .theta.. While all practical reluctance motors have some magnetic saturation, this equation is useful for purposes of the present analysis.
Because torque production in a traditional reluctance machine is almost exclusively a function of the current flowing in the phase winding, the power drives that are coupled to a traditional reluctance machine are required to have a rating corresponding to the highest expected torque output, and thus the highest expected phase current. This requirement often increases the costs of the power drives that must be used in conjunction with a given reluctance machine.
Alternately, when the costs of the power drives is a limiting function, the maximum torque output available from a traditional reluctance machine is limited by the maximum current rating of the available power converter. This current limit effectively limits the maximum available torque output from the machine.
It is an object of the present invention to overcome these and other limitations of traditional reluctance machines by, among other things, providing a reluctance machine system that allows for greater torque production for a given machine/power converter power rating, or conversely, allows the same torque to be produced with a lower phase current level, thus requiring a lower rated, and less costly, power converter. Moreover, it is a further object of the present invention to achieve these results with a machine that is rugged and easy to manufacture.